There are three main types of carbon capture and storage (CCS) technology that could eventually help reduce emissions from power stations and other industrial sites: pre-combustion, post-combustion and oxyfuel.

Post-combustionIn this method, CO2 is separated from the flue gas of the power station by bubbling the gas through an absorber column packed with liquid solvents (such as ammonia). In the most widely used system, once the chemicals in the absorber column become saturated, a stream of superheated steam at around 120C is passed through it. This releases the trapped CO2, which can then be transported for storage elsewhere.

More experimental techniques to scrub CO2 from flue gas without the two-step process include using seawater to absorb the gas and then returning the mixture back to the ocean for long-term storage. But, so far, these methods have proved less efficient and reliable.

OxyfuelWhen coal, oil or natural gas is burned in normal air, CO2 makes up around 3-15% of the waste gas – and separating it out is difficult and energy-intensive. An alternative method is to burn the fuel in pure oxygen. In this environment, virtually all the waste gas will be composed of CO2 and water vapour. The latter can be condensed out while the former can be piped or transported directly to a storage facility.

In the oxyfuel system, the challenge is separating large volumes of air into liquid oxygen, gaseous nitrogen, argon and other trace gases. This process can use up to 15% of the power produced at the station.

Pre-combustionThis method is normally applied to coal-gasification combined-cycle power plants. The coal is gasified to produce a synthetic gas made from carbon monoxide and hydrogen. The former is reacted with water to produce CO2, which is captured, and more hydrogen. The hydrogen can be diverted to a turbine where it can be burned to produce electricity. Alternatively, some of this gas can be bled off to feed hydrogen fuel cells for cars.

One disadvantage of the pre-combustion method is that it cannot be retro-fitted to the older pulverised coal power plants that make up much of the world's installed base of fossil fuel power. It could perhaps be used in natural gas stations, where a synthetic gas is first produced by reacting the methane with steam to produce carbon dioxide and hydrogen. But the economic advantage of this method over post-combustion is yet to be proven.